As is well known, catalytic converters for purifying exhaust gas are mounted on vehicles such as automobiles, in order to remove harmful components such as carbon monoxide, hydrocarbons, nitrogen oxides contained in exhaust gas from engines thereof. FIG. 1 is a cross-sectional view schematically showing an embodiment of a catalytic converter. In this catalytic converter 10, an introduction pipe 16 through which exhaust gas discharged from an internal combustion engine is introduced is connected to one end of a metal casing 11, and a discharge pipe 17 through which the exhaust gas which has passed through a catalyst carrier 12 is discharged outside is attached to the other end thereof. Further, the catalyst carrier 12 is provided inside the metal casing 11 with the intervention of a holding material 13 for a catalyst converter.
Furthermore, an electric heater and a temperature sensor for burning particulates accumulated in the catalyst carrier, namely a honeycomb filter, to recover a filtering function (also referred to as regeneration treatment) may be provided on an exhaust gas introduction side (also referred to as a suction side) with respect to a catalyst carrier, and another pipe for feeding combustion air may be connected thereto, although not shown in the figure. According to such constitution, when the amount of the particulates accumulated in the catalyst carrier 12 increases to result in an increase in pressure drop, the regeneration treatment can be conducted.
The metal casing 11 can be constituted as to divide a cylindrical body into two parts along a longitudinal direction thereof, as shown in FIG. 2. The catalyst carrier 12 around which the holding material 13 for a catalyst converter has been wound is located at a predetermined position in a lower shell 22b, and an upper shell 22a is placed on the lower shell 22b so that a through hole 23a formed in an upper fixing portion 23 is exactly superimposed on a through hole 24a formed in a lower fixing portion 24. A bolt 25 is inserted through the through holes 23a and 24a, and fixed by a nut or the like.
Alternatively, the upper fixing portion 23 and the lower fixing portion 24 may be welded with each other. Further, the metal casing 11 may be a cylindrical body 30 as shown in FIG. 3. Although this requires no assembling work necessary for the metal casing having the two-divided structure as shown in FIG. 2, it is necessary to press the catalyst carrier 12 around which the holding material 13 for a catalytic converter has been wound into the cylindrical body from an opening 31 thereof.
The catalyst carrier 12 is generally a cylindrical honeycomb-like formed article made of, for example, cordierite or the like, on which a noble metal catalyst or the like is carried. It is therefore necessary that the holding material 13 for a catalytic converter has a function of safely holding the catalyst carrier 12 so that the catalyst carrier 12 is not damaged by collision with the metal casing due to vibration or the like during running of the automobile, as well as a function of performing sealing so that unpurified exhaust gas does not leak out through a gap between the catalyst carrier 12 and the metal casing 11. Consequently, at present, as the holding material, there has been mainly used a holding material obtained by forming inorganic fibers such as alumina fibers, mullite fibers or other ceramic fibers into a mat shape having a predetermined thickness using an organic binder. Further, the shape thereof is a planar shape shown in FIG. 4 (A). A convex portion 42 is formed on one end of a tabular main body portion 41, and a concave portion 43 having a shape fittable with the shape of the convex portion 42 is formed on the other end. Then, as shown in FIG. 4(B), the main body portion 41 is wound around an outer peripheral surface of the catalyst carrier 12, and the convex portion 42 and the concave portion 43 are engaged with each other, thereby winding the holding material 13 for a catalyst converter around the catalyst carrier 12.
Examples of generally used organic binder include a rubber, a water-soluble organic polymer compound, a thermoplastic resin, a thermosetting resin and the like. Further, when the holding material 13 for a catalytic converter is too thick, a winding operation around the catalyst carrier 12 and a mounting operation in the metal casing 11 becomes difficult. Therefore, it is necessary to make the holding material thin to some degree. Accordingly, in the general holding materials, these organic binders are used in an amount of 5 to 8% by mass based on the total amount of the holding material, and in an amount of about 10% by mass when used in large amount.
However, recently, the catalyst carrier 12 is heated to nearly 1,000° C. in order to enhance purifying efficiency, so that the above-mentioned organic binder is easily decomposed and burnt down to generate various organic gases such as CO2 and CO. In particular, these gases are generated in large amounts in an early stage of actuation of the catalytic converter. The exhaust gas regulation becomes more and more severe, so that there is a possibility of exceeding a specified value by CO2 and the like derived from the organic binder. Further, recently, although electronic control of engines has progressed, the existence of CO2 independent of the original exhaust gas causes sensors of an exhaust system to produce improper operating signals to adversely affect the electronic control of engines. In order to prevent such a problem, manufacturers conduct burning treatment before shipment to burn down the organic binders. Such burning treatment lays a substantial burden on the makers, and poses an important problem.
It is also conceivable to decrease the amount of organic binder used. However, binding force of the inorganic fibers is weakened by the decreased amount to make the holding material 13 for a catalytic converter thick, which causes a problem of deteriorating assembling properties. Further, problems such as a decrease in strength and an increase in friction coefficient of a casing side surface of the holding material 13 for a catalytic converter are also conceivable by a decrease in organic binder. It has been therefore performed that a surface protective layer such as a film, a tape, a nonwoven fabric or resin coating is provided on the casing side surface of the holding material 13 for a catalytic converter (see JP-A-2001-32710 and JP-A-8-61054). However, the surface protective layer is formed in an amount of 15 g/m2 or more. Accordingly, the organic content exceeds 1% by mass based on the total amount of the holding material only by providing it on the surface. When it is tried to decrease the mass of the protective layer, the strength of the protective layer decreases. Accordingly, trouble such as the occurrence of cracks or breakage in the protective layer occurs in winding.